A variety of approaches have been employed for enlarging a diameter of a wellbore. One approach used to enlarge a subterranean wellbore includes using eccentric and bi-center bits. For example, an eccentric bit with a laterally extended or enlarged cutting portion is rotated about its axis to produce an enlarged wellbore diameter. A bi-center bit assembly employs two longitudinally superimposed bit sections with laterally offset axes, which, when rotated, produce an enlarged wellbore diameter.
Expandable reamers may also be used to enlarge subterranean wellbores and may include blades that are slidably, pivotably or hingedly coupled to a tubular body and actuated by way of hydraulic pressure. The blades of these currently available expandable reamers utilize pressure from inside a tool body of the expandable reamer to force the blades, carrying cutting elements, radially outward to engage the formation material defining a wall of the wellbore. The blades in these expandable reamers are initially retracted to permit the expandable reamer to be run through the wellbore on a drill string and, once the expandable reamer is positioned at a desirable location within a wellbore (e.g., beyond the end of a casing section), the blades are extended so the bore diameter may be increased.